WO1996008068A1

WO1996008068A1 - Selective battery charging system

Info

Publication number: WO1996008068A1
Application number: PCT/US1995/010869
Authority: WO
Inventors: Joseph Patino; Henry A. Bogut; Gustavo D. Leizerovich
Original assignee: Motorola Inc.
Priority date: 1994-09-02
Filing date: 1995-08-24
Publication date: 1996-03-14

Abstract

A lock-and-key system for selectively enabling and disabling a battery charger is provided. The system lock is provided by a Hall effect switch (140) which, when activated, will allow a battery charger (102) to properly charge a battery (106) loaded thereinto. The key for activating the Hall effect switch is a magnet (142) adapted to apply a magnetic field to the Hall effect switch (140). In embodiments in which the Hall effect switch is disposed in the charger, the magnet is disposed in the battery pack. Conversely, when the Hall effect switch is disposed in the battery pack, the magnet is provided in the battery charger.

Description

SELECTIVE BATTERY CHARGING SYSTEM

Technical Field

This invention relates in general to battery charging systems, and more particularly to battery charging systems which are selectively enabled or disabled depending upon the battery placed into the system.

Background

It is very common for battery packs which are used in portable communications devices, such as two-way radios or cellular telephones to have a thermistor and a battery capacity resistor. The thermistor is used during battery charging to determine if the battery is being charged properly. The capacity resistor determines the capacity of the battery prior to the battery being charged. The battery charger, upon determining the battery capacity, will select the proper charging rate to optimally charge the battery pack. As virtually all battery packs and battery chargers have a thermistor and a battery capacity resistor, almost all batteries can be placed interchangeably into different battery charging systems. As a result, newer batteries having different battery chemistries and requiring particular charging algorithms may be erroneously placed into battery chargers, damaging the batteries, and potentially resulting in catastrophic failure of the battery pack and battery charging system.

Referring now to FIG. 1, there is shown a prior art battery charging scheme consisting of a charger (102), a radio battery pack (106), and an electronic device, such as a radio (104). Radio (104) contains positive (B+) and negative (B-) battery terminals which are coupled to radio battery (106) via battery contacts (116 and 114) respectively. Battery (106) contains one or more battery cells (108) which determine the voltage and current capacity of the battery (106). Also included as part of the battery (106) is a protection diode (118) a battery temperature indicator, such as thermistor (Rt) (112) and a battery capacity indicator such as resistor (Re) (110).

Charger (102) consists of a charger monitor circuit (128) which consists of a microprocessor or microcontroller as is known in the art, along with appropriate control software, also known in the art. Charger monitor circuit (128) controls charger control circuit (130) which provides current battery (106) in order to charge the battery. Control signals are transmitted by charger monitor circuit (128) to charger controller circuit (130) via bus (140). The control signal informs charger control circuit (130) on how much current to source via line (129) to battery (106).

Charger monitor circuit (128) contains three analog to digital (A/D) ports (120, 122, and 124), and one ground port (126). A/D port (120) monitors the voltage on the B+ line (132). A/D port (122) senses the resistance of capacity resistor Re (110) via line (134) and A/D port (124) senses the resistance of thermistor Rt (112) along lines (136), as its resistance changes once the battery begins charging. B- (114) is connected to ground port (126) via lines (138) A/D ports (122, and 124) include external pull-up resistors (123 and 125 respectively) which are used to determine the resistance of Re (110) and Rt (112) by determining the voltage level at A/D ports (122) and (124) respectively.

As noted above, the problem with the prior art charging system shown is that it allows any type of battery pack satisfying the form factor requirements to be plugged into the charging system regardless of the type of battery cells (108) incorporated into the battery pack (106). As a result, a battery charger. (102) adapted to charge, for example, a nickel cadmium cell in a rapid charging mode, may erroneously apply such rapid charging to a lithium ion cell yielding catastrophic results. Less dramatic, though equally damaging results, would be obtained from improperly charging a nickel cadmium or a nickel metal hydride charger in systems adapted to charge lithium or lithium ion batteries. In such a situation, the nickel metal hydride or nickel cadmium batteries will be improperly charged, thus obtaining only a partial charge at best. Repeated charging on an inappropriate charger will dramatically shorten the cycle life of a nickel cadmium or nickel metal hydride battery while reducing significantly the amount of charge stored thereon. Both situations yield unacceptable results to consumer demands for batteries being long cycle life and extended usage periods. Accordingly, there exists a need to provide a battery charging system which will be selectively enabled or disabled to allow for charging only specific types of batteries. Means for selectively enabling and disabling the charging system should be relatively small so as to not unduly enlarge the size of the battery pack or charging system, as well as economic and reliable. Brief Description of the Drawings

FIG. 1 is a schematic representation of a prior art battery charging system; FIG. 2 is a battery charging system in accordance with the instant invention;

FIG. 3 is a second embodiment of a battery charging system in accordance with the instant invention; and

FIG. 4 is a third embodiment of a battery charging system in accordance with the instant invention.

Detailed Description of the Preferred -Embodiment

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.

Referring now to FIG. 2, there is illustrated therein a battery charging system in accordance with the instant invention. For ease of explanation, only those added features provided by the present invention over the prior art charging system shown in FIG. 1 will be discussed. In concept, the instant invention provides a "lock-and-key" system by which only certain batteries may be charged in the charging system. The "lock" in this instance is a Hall effect switch while the "key" is a magnet. The lock may be electrically coupled in either the charger (102) or the battery (106). Accordingly, the "key" is disposed in either the battery or the charger depending upon the location of the lock. However, the lock and key should be placed in separate elements. A Hall effect switch takes advantage of the Hall effect, where if a current carrying conductor is placed in a magnetic field and oriented so that the field is at right angles to the direction of the current, an electric field is produced in the conductor at a right angel to both the current and the magnetic field.

Referring now to FIG. 2, there is illustrated therein a Hall effect switch - the lock - disposed in the battery charger (102). The switch (140) is electrically coupled to Re line 134. The battery (106) thus includes the magnet (142) which acts as the key. Thus, the magnet (142) turns on the Hall effect switch (140) in the charger. If the magnet is present in the 4 battery, the Hall effect switch (140) will switch the 5V supply onto the Re line through the pull-up resistor ( 123). This will cause the charger to be able to read both the Re value and Rt value of the battery. In this mode, the charger will rapidly charge the approved battery normally and according to the charging instructions stored in the charger control circuit (130). If the battery is not an approved battery, (i.e., does not include a magnet to enable the Hall effect switch), the charger's Hall effect switch will be in the off mode and the charger will only be able to read the battery's Rt value; the charger will see 0 volts on the Re line. In this mode, the charger may be programmed to recognize that the battery present is not approved, and can either not charge the battery at all, or simply trickle charge the battery at a level safe for any battery chemistry.

Referring now to FIG. 3, there is illustrated therein a second embodiment to the battery charging system in accordance with the instant invention. In this embodiment, the "lock" -- the Hall effect switch (140) is disposed in the battery (106) while the "key" -- the magnet (142) - is disposed in the _.charger (102). In the embodiment of FIG. 3, the charger (102) further includes a switch (144) adapted to switch between two voltage levels, for example, +5 volts and a second voltage designated A+. The switch (144) is electrically coupled to the Hall effect switch (140) via Re line (134). If a battery with the Hall effect switch (140) incorporated therein is loaded into a battery charger (102) without the magnet (142) present therein, the Hall effect switch will not be turned on. Accordingly, the charging path will not be turned on nor will the charger be able to read the Re value. In this mode, the charger will either not charge the battery at all or will only trickle charge at a level safe for all battery chemistries.

Conversely, if the battery (106) is loaded into an appropriate charger (102) including a magnet (142), the Hall effect switch (140) will be turned on, and the charger can determine the battery type by switching the 5- volt supply onto the Re line. The voltage at the A/D port (122) is now the resistor divided voltage of the pull-up resistor (123) and the Re resistor (110). This voltage will be offset slightly by the Hall effect switch's collector to emitter voltage. This offset can be easily compensated for in the charger. Once the Re is determined, the charger can enable charging by switching the A+ supply onto the Re line to bias field effect transistor (FET) (146) into the on condition. This will occur since the base of a PNP transistor (148) is pulled into the low condition by the Hall effect switch (140) thus switching the A+ supply onto the gate of FET (146). The supply on the gate of the FET (146) needs to be maintained above the source by a significant amount to assure that the FET remains properly turned on. Once turned on, the on resistance for the FET is approximately 0.035 ohms so that at 2 amps the voltage drop is reduced to 0.07 volts for a total of 140 mW. A feature of the configuration is the use of the Re resistor to act both as the capacity resistor to be read by the charger to determine the battery type, and also to bias transistor (148).

Referring now to FIG. 4, there is illustrated therein a battery charging system in accordance with the instant invention. In this embodiment, the need for the additional switch (144) and the PNP transistor (148) illustrated in FIG. 3, is obviated; however, the charger will be able to read the Re of the battery. In the embodiment of FIG. 4, the source and drain of FET ( 146) are reversed as compared to that of the embodiment of FIG. 3. In this configuration referred to hereinafter as the P Channel FET approach, the P Channel on resistance is 0.1 ohms, so that at the 2 amps, the voltage drop is 0.24 voltages (480 mW). Although the on resistance is not as low as in the embodiment illustrated in FIG. 3, the P Channel approach requires no additional 5 volt/A+ supply switching. The FET (146) is simply not allowed to turn on because the resistor (150) keeps the voltage at gate and source equal unless the Hall effect switch (140) is turned on to pull the gate of the FET (146) low. Once the gate is pulled low by the Hall effect switch, the FET (146) is turned on and current is allowed to pass through to the battery. In this configuration, the charger is allowed to read the Re, but not allowed to charge the battery if the magnet is not present in the charger to turn on the Hall effect switch.

In summary, the present invention provides a "lock-and-key" for assuring that a proper battery pack/charger system, thus avoiding destructive or otherwise deleterious effects resulting from inappropriate charging. The lock-and-key approach is characterized by the use of a Hall effect switch in either the charger or the battery and a magnet for enabling the Hall effect switch disposed in the element in which the Hall effect switch is not disposed. The present invention yields a solution which requires minimal design changes to existing charger systems, yet provides for selective charger enablement and disablement depending upon the batteries loaded thereinto. While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A battery charging system comprising: a battery pack having a position and a negative terminal, and at least one battery cell disposed therein; a battery charging device adapted to apply a charging voltage to a battery disposed therein; and means for magnetically enabling said battery charging device when a battery pack is resident therein.

2. A battery charging system as in claim 1, Wherein said means for magnetically enabling said battery charging device comprises: a Hall effect switch; and a magnet for suitably said Hall effect switch.

3. A battery charging system as in claim 2, wherein said Hall effect switch is electronically coupled to a charger monitor circuit in said battery charging device, and said magnet is resident in said battery pack.

4. A battery charging system as in claim 2, wherein said Hall effect switch is electrically coupled between said battery pack position and negative terminals, and said magnet is resident in said battery charging device.

5. A battery charging system as in claim 4, wherein said battery pack further includes a field effect transistor having a source, a gate, and a drain, and wherein said source and drain are electrically coupled to said positive terminal and said gate is electrically coupled to said Hall effect switch.

6. A battery charging system as in claim 4, wherein said Hall effect switch is electrically coupled to a sensing resistor disposed in said battery pack.

7. A battery charging system comprising: a battery pack having a positive and negative terminal, and at least one battery cell and a magnet disposed therein; and a battery charging device including a Hall effect switch disposed therein, said battery charging device adapted to apply a charging voltage to said battery pack.

8. A battery charging system as in claim 7, wherein said Hall effect switch is electrically coupled to a charger monitor circuit in said battery charging device.